Reversible phosphorylation of protein hydroxyl groups is a ubiquitous signaling mechanism that renders a limited set of genes capable of cellular specialization and differentiation. In humans, at least 518 genes encode protein kinases and over 100 genes encode protein phosphatases, in total accounting for approximately 2.5% of all human genes [1]. Protein phosphorylation is central to cellular regulation with roles in, for example, cell signaling, signal transduction, and mitosis. Changes in protein phosphorylation states and kinase activity are also associated with many human diseases, most notably cancer, Alzheimer's disease, and heart disease [1-2].
A major class of emerging pharmaceuticals are kinase inhibitors, which typically are small molecules that block the actions of kinases (e.g., Gleevec® (imatinib mesylate) used in cancer treatments; VX-680, an Aurora kinase inhibitor in clinical trial; Herceptin® (trastuzumab), for treatment of breast cancer; Avastin® (bevacizumab), for treatment of metastatic colorectal cancer; Iressa® (gefitinib) and Erbitux® (cetuximab), for treatment of lung and colorectal cancer; everolimus, for treatment of Metastatic Breast Cancer). The vast majority of kinase inhibitor pharmaceuticals or compounds in clinical trial are tyrosine kinase inhibitors; thus, a need exists for identifying and developing novel inhibitors of serine and threonine kinases, which play important roles in cellular and extracellular functions.
Protein phosphorylation is thus a critical regulatory strategy. New tools are necessary which may be used to interrogate and are responsive to the activities of protein kinases and phosphatases.